1. Field of the Invention
The present invention relates to electronic assemblies, and in particular, to electrically and mechanically connecting two electronic components to form an electronic assembly having reduced thermo-mechanical fatigue, while also providing the electronic assembly with the ability to use existing bond and assembly technologies in addition to providing the assembly with reworkability.
2. Description of Related Art
Forming electronic assemblies by electrically connecting two components such as a multi-layer ceramic package or circuit chip to a card, board, or another connector is well known in the art. Such multi-layer electronic assembly formation is referred to in the art as surface mount technology which can include, but is not limited to, solder join, flip chip, C4, ball grid array and pin grid array. As will be recognized, surface mount technology has gained acceptance as the preferred means of joining electronic package assemblies, particularly in high end computers.
Over the years, surface mount technology has been accomplished by a variety of techniques such as, for example, the use of interposers, soldering techniques, conductive adhesives, and the like. Prior art is directed to using thermoplastic interposers, typically a fully cross-linked high modulus material with very low compliance, having a plurality of cure stages for use in surface mount technology whereby the interposer can be positioned between any two mating surfaces having connector arrays. The interposer may be provided with holes which can be filled with solder or a conductive adhesive to connect the two substrates to the interposer. Typically, the interposer is then permanently bonded to both components through heat and pressure. However, as a result of the interposer comprising a thermoplastic polymer, the interposer is typically made of a high modulus non-compliant material which does not alleviate CTE mismatch induced strain through material movement. The adhesion of the interposer to both electronic components of the assembly typically induces a strain gradient across the interposer. In an assembly having a strong bond between the interposer and the electronic components, the interposer may influence the CTE related movement of each component and thus minimize both the difference in material movement and the strain, potentially resulting in an electronic assembly having increased working life. However, such technology relies on strong, typically permanent, bonds between the interposer and both electronic components, and thereby is not compatible with electronic assemblies requiring or desiring the ability of component reworkability.
In addition to the introduction of interposers for use in surface mount technology, numerous solder structures have been proposed for surface mounting. Typical surface mount processes form solder structures by screening solder paste onto conductive pads exposed on the surface of the first electronic substrate. The solder paste is reflowed typically in a hydrogen atmosphere and homogenizes the pad and brings the solder into a spherical shape which is then aligned to corresponding pads on the electronic structure or board to be connected thereto. After alignment, the substrate and board go through a reflow operation to melt the solder and create a solder bond between the corresponding pads on the substrate and other electronic component. However, such soldering techniques produce solder bonds of very small height leading to decreased strain absorption capability, as well as costly high temperature reflows as a result of the double reflow operations.
Semiconductor chips and multilayer ceramic or organic electronic components are also joined together by Controlled Collapse Chip Connection on a surface of one of the electronic components to corresponding pads on the surface of the other component. Controlled Collapse Chip Connection (C4) is an interconnect technology developed by IBM as an alternative to wire bonding. C4 technology provides a more exact and somewhat greater quantity of solder to be applied than can be applied through screening. In the C4 interconnect technology, a relatively small solder bump or solder ball is attached to pads on one of the components being joined, therein the conventional chip joining technology providing for semiconductor chip and ceramic or organic substrates to be attached to each other. The electrical and mechanical interconnects are then formed by positioning the corresponding pads on the other electronic component adjacent the solder bumps and reflowing the bumps at an elevated temperature. The C4 joining process is self-aligning in that the wetting action of the solder will align the chip bump pattern to the corresponding substrate pads.
Further techniques for mechanically and electrically connecting various components in surface mount technology include providing a module with a ball grid array (BGA) of solder balls In such techniques, balls of solder are arranged in a predetermined pattern on the module corresponding to a pattern of attachment pads on a substrate, typically referred to as a footprint. The solder balls of the module may then be aligned to the attachment pads of the substrate. Typically, a solder paste may be applied to the attachment pads on the substrate to provide the flux required and also cause the solder balls to adhere to the attachment pads to maintaining alignment with the solder balls during heating and reflow of the solder to form the assembly.
C4 technology (chip to substrate connection) and BGA technology (substrate to board) offer advantages of low cost, high I/O density, low inductance surface mounting interconnection, potentially smaller packages, as well as robust processing steps. However, these area array technologies are limited by strain absorption due to the allowable diameter of the solder sphere. Typically, C4 bumps are 3-5 mils in diameter while BGA solder balls are approximately 20-35 mils in diameter, whereby the size of the bump or solder ball determines the fine pitch capability, as well as the strain absorption of the interconnect with a coefficient of thermal expansion (CTE) mismatch between the chip and substrate, or substrate and board. For example, the CTE mismatch may be about 15 ppm/xc2x0C. for a glass-ceramic module on an FR-4 (fiber-reinforced epoxy) card.
During normal operations, the entire module is subject to temperature excursions due to the functioning of the circuits on the chip, resistance heating of the solder joints, the wiring within the chip, and the wiring within the substrate. This heating results in the expansion and contraction of all of these components as temperatures rise and fall. Chips are primarily comprised of silicon, which has a coefficient of thermal expansion in the range of about 3.0 ppm/xc2x0C. The corresponding substrates to which the chips are joined are typically made of ceramic or organic materials, which have coefficients of thermal expansion in the ranges of about 3 to about 7 ppm/xc2x0C. and about 12 to about 20 ppm/xc2x0C., respectively, while the corresponding printed circuit board typically has a coefficient of thermal expansion in the ranges of about 15 to about 22 ppm/xc2x0C. As a result, the chip and the substrate, and substrate and card expand and contract at different rates during thermal cycling. This mismatch places stresses on the solder joints, and over time results in the fatigue of the solder joints. Eventually, continual stress causes cracks to propagate completely across the solder joints leading to electrical failure of the electronic module.
Multilayer ceramic electronic components are also joined to printed circuit boards using high melt columns in place of the spheres which are joined to the corresponding metal pads on the surface of each component with a lower melting solder. Column grid array (CGA) technology permits greater strain absorption than conventional BGA due to the increased joint height, while simultaneously allowing the capability of finer pitch. Thus, the greater strain absorption of CGA technology provides for a more reliable and extendable electronic assembly than those assemblies formed by conventional BGA technologies. However, CGA technology is typically used on larger substrates that require an interconnection that can withstand a greater amount of strain. Furthermore, CGA adds considerable cost, electrical properties are compromised, and manufacturing complexities are increased as a result of solder columns being easily bent during handling thus requiring special precautionary measures during processing.
Prior art is also directed to the use of conductive adhesives for joining components in surface mount technology. Over the years, conductive adhesives have replaced the use of conventional soldering material, such as PbSn, in solder structures to join electronic components together in surface mount technology. However, it has been recognized that conductive adhesives are not as mechanically strong as solder joints causing failure under certain mechanical shock conditions when used as electrical interconnections. Conductive adhesive bonds, which typically have a silver filler, are not a metallurgical bond as formed by the solder joints and thus are not as strong as the solder joint bond. Further problems associated with conductive adhesive joints in electronic packaging include an increased contact resistance, oxidation of silver flakes, and silver migration may occur with temperature humidity and bias. Also, the formation of metal oxide, hydroxide, and other corrosion byproducts at the interface between a conductive adhesive and the metal bonding surface may compromise both the electrical and mechanical stability of the adhesive bonds, and thus the performance and reliability of the resultant electronic assembly.
As prior art is directed to surface mount technology for connecting two components together to form an electronic assembly including the use of interposers, soldering techniques, and conductive adhesives, such techniques have their corresponding problems as discussed above. Thus, as surface mount technology progresses, and smaller electronic assemblies are required, a need continues in the art to provide improved methods and surface mount technologies for mechanically and electrically connecting two components together to form an electronic assembly having enhanced reliability.
Bearing in mind the problems and deficiencies of the prior art, it is therefore an object of the present invention to provide an apparatus and method of connecting two components to form an electronic assembly having the structural stability of an interposer, while having greater absorption of CTE mismatch induced strain, with the exact attachment location capability as provided with BGA technologies in combination with greater strain absorption than conventional BGA technologies.
It is another object of the present invention to provide an apparatus for forming an interposer structure between two interconnected substrates of an electronic module to enhance the mechanical and electrical integrity and reliability of the module.
A further object of the invention is to provide electronic modules having solid conductive joints with enhanced mechanical strain absorption and reliability.
Another object of the present invention is to provide a circuit board capable of receiving different chip modules at each chip module receiving site.
It is yet another object of the present invention to provide an electronic component assembly or module made using the method and apparatus of the invention.
Another object of the invention is to provide improved surface mount technology for use in connecting smaller multi-chip modules having structural stability with greater strain absorption for a more reliable electronic assembly.
Still another object of the present invention is to provide an electronic component assembly or module having reducing silver migration.
It is another object of the present invention to provide a circuit board which provides reworkability between the joined substrates.
Yet another object of the present invention is to provide an electronic component assembly having reduced thermo-mechanical fatigue.
Another object of the invention is to provide an electronic component assembly with the ability to use existing bond and assembly technologies.
Still other objects and advantages of the invention will in part be obvious and will in part be apparent from the specification.
The above and other objects and advantages, which will be apparent to one of skill in the art, are achieved in the present invention which is directed to, in a first aspect, a method for electrically connecting two components by providing an insulating matrix with a plurality of conductor holes. Preferably, the insulating matrix comprises a polymer insulating matrix with the plurality of conductor holes formed in the insulating matrix to match corresponding contact arrays of the first and second substrates on first and second surfaces of the insulating matrix. The plurality of conductor holes may be formed by laser ablation, mechanical punching, drilling, or may be provided within the insulating matrix as-formed by injection molding or transfer molding techniques.
The insulating matrix may comprise a low modulus insulating matrix comprising a low modulus material selected from the group consisting of silicone, silicone epoxy, urethane, and flexible epoxy. Alternatively, insulating matrix may comprise a high modulus insulating matrix comprising a high modulus material selected from the group consisting of epoxy, polysulfone, and polyimide siloxane. The insulating matrix may also comprise a B-staged thermoset insulating matrix or a thermoplastic insulating matrix. The insulating matrix may then be aligned with the first substrate, whereby the plurality of conductor holes on a first side of the matrix align with contact arrays on the first substrate.
A conductive material, preferably a conductive adhesive, is then provided within the plurality of conductor holes. By providing the conductive material into the conductor holes in the matrix, the insulating matrix provides structural support to the conductive material within the plurality of conductor holes. The conductive material may comprise a flexible, low modulus conductive adhesive, or a high modulus conductive adhesive. In the present invention, preferably the insulating matrix and the conductive adhesive comprise similar materials thereby the conductive adhesive absorbing a CTE mismatch strain of the electronic assembly.
Subsequently, a solid conductive material is attached to an end of the conductive adhesive within the conductor holes in the insulating matrix. Preferably the solid conductive material comprises a solid conductive metal material. In the present invention, the solid conductive metal material may comprise copper, brass, nickel, tin, gold, lead, and combinations thereof. Furthermore, the solid conductive metal material may be a variety of contacts including a pin, cone, stud, ball, and disk. In attaching the solid conductive material to the conductive adhesive a portion of the solid conductive material may be provided within the plurality of conductor holes in the matrix. In the preferred embodiment, the a portion of about 25% to about 75% of the solid conductive material may be provided within the plurality of conductor holes.
Once the solid conductive material is positioned in alignment with the conductive adhesive, a conductive matrix structure is then formed. The conductive matrix structure comprises the insulating matrix completely surrounding the conductive adhesive which has attached thereto the solid conductive material. The conductive matrix structure may then be permanently secured to the first substrate by heating the conductive matrix structure to a temperature sufficient to completely cure the conductive matrix structure and permanently bond the same to the first substrate.
After the conductive matrix structure is permanently secured to the first substrate, the connection of the resultant assembly adapted with reworkability may be formed by connecting the structure to a second substrate. The first substrate having attached thereto the insulating matrix with the solid conductive material attached to the conductive material may be secured to the second substrate by an attachment means which adapts the resultant electronic assembly with reworkability. In the preferred embodiment of the present invention, the attachment means comprises a solder material which adapts the resultant electronic assembly with reworkability while simultaneously maintaining substrate integrity.
In yet a further aspect, the present invention is directed to an electronic assembly made by the method as described above and further below.